The present invention relates to hardcoatings primarily for use in wear applications and methods for fabricating same by thermal spraying of powder feeds and more particularly to multimodal coatings which are fabricated by thermal spraying aggregated micro/nano-scale feedstock powders.
Materials with fine-scale structures have long been recognized to exhibit remarkable and technologically attractive properties. Over the past decade, interest has been growing in a new class of materials known as nano-scale materials, that are composed of ultra-fine grains or particles. A feature of nano-scale materials is the high fraction of atoms that reside at grain boundaries or interfaces in the materials, which can have a beneficial effect on properties. Although much of today""s research and development activity is focused on the synthesis and processing of nano-scale bulk materials, there is a growing interest in the fabrication of nano-scale coatings.
Conventional WC/Co powders, having 3-30 wt. % Co, are routinely produced by mechanically milling micron-scale powders of phase pure WC and Co. The difficulty associated with uniformly mixing such powders by mechanical means has heretofore limited the WC grain size of the product powder to about 0.1-0.3 micron. More recently, an alternative chemical processing technology, known in the art as Spray Conversion Processing (SCP) has been developed for producing nano-scale WC/Co powder as described in U.S. Pat. No. 5,230,729, U.S. Pat. No. 5,651,808, and U.S. Pat. No. 5,919,428, all of which are assigned to Nanodyne, Inc. (now Union Miniere). In SCP, two phases are ideally mixed at a scale less than 0.1 micron. In addition to nanophase WC/Co, SCP enables a wide variety of other technically important nano-scale metal carbide/metal systems to be produced, such as Cr3C2/NiCr and TiC/Fe.
The SCP production method involves essentially three sequential steps: (1) aqueous solution mixing of ammonium metatungstate (source of tungsten) and cobalt acetate (source of cobalt) to fix the composition of a starting solution, (2) spray drying the starting solution to generate a chemically homogeneous precursor powder, and (3) fluid-bed thermochemical conversion (pyrolysis, reduction and carburization) of the precursor powder to a nano-scale WC/Co powder end-product. A feature of the asynthesized nano-scale powder is its characteristic spherical-shell morphology, which is obtained via the spray drying step of the process. Another feature is that each shell-like or hollow particle is highly porous in nature, and is composed of a completely uniform mixture of WC and Co nano-scale grains, typically with a WC grain size of about 30 nm.
Recent work has demonstrated that asynthesized nano-scale WC/Co powder, can be used directly as feedstock material in thermal spraying.
Alternatively, the powder can be re-processed into solid agglomerates, as taught in U.S. Pat. No. 6,025,034, which gives some advantages in terms of coating quality, e.g. reduced porosity. Powder re-processing involves mechanically milling the asynthesized nano-scale WC/Co powder in an appropriate fluid medium, which breaks up the shell-like particles (10-30 micron diameter) into much smaller fragments (0.1-1 micron diameter). The slurry formed by milling is spray dried to form solid nano-scale particle agglomerates (5-40 micron diameter). In current practice, asynthesized nano-scale WC/Co powders (hollow particles or solid agglomerates) are used as feedstock powders in high velocity oxy-fuel (HVOF) spraying of nano-scale WC/Co coatings. Unfortunately, asynthesized nano-scale WC/Co powders can not be used as feedstock powders in plasma spraying because plasma spraying causes extensive decarburization of the WC phase, due to the much higher temperatures achieved during spraying.
Oxide-ceramic powders with fine and ultra-fine particles sizes have traditionally been produced by solution/precipitation methods, however, recent development work has focused on vapor condensation methods. Pioneering work in this field has resulted in the Inert Gas Condensation (IGC) process. In this process, which is conducted in a low pressure (typically 1-30 mbar) inert gas environment, nano-scale particles are produced in a thermalizing zone just above the evaporative source, due to interactions between the hot vapor species and the much colder inert gas atoms in the chamber. The particles are convectively transported to and collected on a water-cooled copper plate. Today, the ICG process is widely used in the production of metal powders, and some metal-oxide powders.
In the past few years, significant modifications to the basic ICG process have been made in order to produce nano-scale powders from readily available metalorganic precursors. The modified ICG, called Chemical Vapor Condensation (CVC), a precursor/carrier gas stream is decomposed in a hot-wall or combustion-flame reactor to generate a stream of nano-scale particles exiting the hot zone of the reactor as described in U.S. Pat. Nos. 5,514,350 and 5,876,683. The CVC process has been used to produce nano-scale ceramic powders, which cannot easily be produced by the IGC process, because of the high melting points and/or low vapor pressures of the starting materials. Both hot-wall and combustion-flame reactor systems are now being developed for high rate powder production of oxide and non-oxide ceramic nano-scale powders. Product powders available today include Al2O3, TiO2, ZrO2, SiC, Si3N4, AlN and mixtures thereof.
Comparison tests, using both conventional and nano-scale Al2O3/TiO2 feedstock powders, have shown that coatings obtained by plasma spraying of nano-scale powders display a modest increase in hardness, but a significant improvement in wear properties.
Accordingly, improved nano-scale ceramic/metal and ceramic/ceramic coatings and methods for making same are needed for wear applications.
A method for producing an abrasion resistant coating composed of a ceramic/metal material system or a ceramic/ceramic material system, the method comprising the steps of: blending micron-scale particles of a hard phase material with nano-scale particles of a binder phase material to form a uniform powder mixture; aggregating of the powder mixture to bond the nano-scale particles to the micron-scale particles thereby forming a feedstock powder comprised of particle aggregates; and thermal spraying the feedstock powder of particle aggregates onto a substrate thereby forming the abrasion resistant coating thereon. Coatings produced by this method are composed of micron-scale particles of the hard phase material fused together with the binder phase material.